1) Field of the Invention
The present invention relates to an electromagnetic shield structure for electromagnetically shielding a circuit board substrate. More particularly, the present invention relates to an electromagnetic shield structure that can block electromagnetic noise radiating from a circuit board substrate.
2) Description of the Related Art
Occurrence of electromagnetic noise when circuit elements are powered up is a common phenomenon in a circuit board substrate on which circuit elements such as transistors, IC, LSI, etc. are mounted. If the electromagnetic noise is ignored and no appropriate action is taken to shield it, it may lead to the malfunctioning of the circuit board substrate, the electronic gadget in which the circuit board substrate is used, and/or other electronic gadgets.
There are two approaches to counter this problem. One is to see to it that no major electromagnetic noise occurs in the first place. The other is to suppress the electromagnetic noise, if there is any, so that there is no widespread diffusion outside.
The former approach is a question of selection of the circuit. There are technologies those make it possible to drastically reduce the electromagnetic noise, for instance, see Japanese Patent Laid-Open Publication No. H10-270862. In the technology disclose in this literature, a multi-layer printed circuit board substrate is fabricated by employing the method of assembling on the printed circuit board substrate the circuit elements such as the IC, LSI, etc. and wiring of the power source layer (pattern design).
In the latter approach, a method involving shielding, that is, using an appropriate material to enclose the circuit board substrate so as to block the radiating electromagnetic noise, may be used. Conventional shielding method involves sealing both the surfaces of the circuit board substrate by a metal frame in order to block the electromagnetic noise.
However, the conventional method of sealing both surfaces of the circuit board substrate with a metal frame requires more area because the frame needs to be accommodated. In addition, the cost is higher since the metal frame adds to the product cost.